After all that trouble, the (hopefully working) prototype for the board arrived yesterday. It looks great, though work on it will have to wait a bit what with various other things going on in my life. The one exception will, I think, be soldering on some stacking headers and getting it into a stack so it can be tested.
This will be fine for regular Arduino projects, I think, though for the wearable voice changer project that this is supposed to be the core for. I'll probably still work to perfect this, though I think I may also work on dividing the basic audio I/O and the FV-1 processing back into two separate functions, but on sewable boards (rather than the Feather Wings that I started all this with).